E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A shrub of the Vitex tribe, called agnos (chaste) by the Greeks, because the Athenian ladies, at the feast of Cers, used to strew their couches with vitex leaves, as a palladium of chastity. The monks, mistaking agnos (chaste) for agnus (a lamb), but knowing the use made of the plant, added castus to explain its character, making it chaste-lamb. (For another similar blunder, see I.H.S.)